1. Field of the Invention
The present invention relates in general to an electro-optical device such as a ferroelectric liquid crystal device. More particularly, it relates to such a ferroelectric liquid crystal device improved with respect to driving characteristics.
2. Description of the Prior Art
As conventional liquid crystal displays, displays utilizing twisted nematic liquid crystal materials are well known in the field. The displays of this type often exhibit cross-talk between adjacent pixels when driven in matrix configuration having a large number of pixels, so that the available pixel number is substantially limited.
Displays of the active matrix type provided with thin film transistors for driving respective pixels are also known. Fabrication of the displays of this type, however, may present difficulties owing to low yield of transistors formed on the substrate particularly when the size of the substrate is increased. The need for relatively large equipment investment also make it difficult to proceed with this type.
A new type display device utilizing ferroelectric, chiral smectic C liquid crystal has been suggested by N. A. Clark et al, in U.S. Pat. No. 4,367,924. In this device, the smectic liquid crystal material is layered, and the layers 12 are aligned perpendicular to opposed surfaces 11 and 11' of the cell as shown in FIG. 1. The liquid crystal molecules lie flat on the surfaces and are restricted at the surface to only two positions (i.e. first and second states (I) and (II) in which the liquid crystal molecules are inclined at .theta. and -.theta. from the layer normal) out of the cone of possible orientations that the chiral smectic state allows as illustrated in FIG. 9. The surfaces of the cell have to be closely spaced so that the bulk of the sample follows the molecular orientation at the surface, thereby creating the two surface stabilized states. The influence of the surfaces also helps to suppress the helix of the chiral smectic material so that the two states are not disrupted. The dipole moment of spontaneous polarization, points normal to the surfaces, up in the first state and down in the second state for example. Hence, the device can be switched between the two states with a pulsed electric field applied via an electrode on the surface. The influence of the closely spaced surfaces causes the switched state to latch so that the state is maintained after the pulse is over, indicating memory characteristics.
In such a ferroelectric liquid crystal device, it is required to accomplish uniform driving performance throughout the entirety of the device, so that efforts have been made to obtain a liquid crystal layer having a uniform liquid crystal orientation with no defects throughout the entire device, i.e. a mono-domain of the liquid crystal.
The mono-domain, however, cannot be formed due to defects in the ferroelectric liquid crystal caused by small flaws occurring in an orientation film, stepwise unevenness of the electrodes for driving the liquid crystal, spacers provided for maintaining the appropriate gap between the substrates or other causes. An approach to form the mono-domain is to let crystals grow in one direction by virtue of a gradient of temperature. This approach can not be applied for the case of large devices in industrial production, but only for the cases of small devices of about several square centimeters.
Even if a mono-domain can be formed, the layered structure of the liquid crystal often incurs zig-zag defects because the ferroelectric liquid crystal is inherently not aligned in parallel to the substrate to form alignment thereof inclined at a certain angle, so that bending or displacement is caused. For this reason, displaying and driving characteristics become non-uniform.